This invention relates to a method and apparatus for separating air.
The most important method commercially of separating air is by rectification. The most frequently used air separation cycles include the steps of compressing a stream of air, purifying the resulting stream of compressed air by removing water vapor and carbon dioxide, and pre-cooling the stream of compressed air by heat exchange with returning product streams to a temperature suitable for its rectification. The rectification is performed in a so-called "double rectification column" comprising a higher pressure and a lower pressure rectification column i.e. one of the two columns operates at higher pressure than the other. Most if not all of the air is introduced into the higher pressure column and is separated into oxygen-enriched liquid air and liquid nitrogen vapor. The nitrogen vapor is condensed. A part of the condensate is used as liquid reflux in the higher pressure column. Oxygen-enriched liquid is withdrawn from the bottom of the higher pressure column, is sub-cooled, and is introduced into an intermediate region of the lower pressure column through a throttling or pressure reduction valve. The oxygen-enriched liquid is separated into substantially pure oxygen and nitrogen products in the lower pressure column. These products are withdrawn in the vapor state from the lower pressure column and form the returning streams against which the incoming air stream is heat exchanged. Liquid reflux for the lower pressure column is provided by taking the remainder of the condensate from the higher pressure column, sub-cooling it, and passing it into the top of the lower pressure column through a throttling or pressure reduction valve.
Conventionally, liquid oxygen at the bottom of the lower pressure column is used to meet the condensation duty at the top of the higher pressure column. Accordingly, nitrogen vapor from the top of higher pressure column is heat exchanged with liquid oxygen in the bottom of the lower pressure column. Sufficient liquid oxygen is able to be evaporated thereby to meet the requirements of the lower pressure column for reboil and to enable a good yield of pure gaseous oxygen product to be achieved.
An alternative to this conventional process is to use a part of the feed air to provide the necessary heat to reboil liquid in a first reboiler-condenser at the bottom of the low pressure column. This alternative removes the link between the top of the higher pressure column and the bottom of the lower pressure column. Accordingly, the operating pressure ratio between the two columns can be reduced, thus reducing the energy requirements of the air separation process. Nitrogen separated in the higher pressure column is condensed in a second reboiler-condenser by heat exchange with liquid withdrawn from an intermediate mass-exchange region of the lower pressure rectification column. This alternative kind of process is referred to as a "dual reboiler" process.
One disadvantage of dual reboiler processes is a difficulty in obtaining an argon product by rectification of an argon-enriched oxygen stream withdrawn from the lower pressure rectification column. In order to produce such an argon product effectively, it is desirable to operate the bottom section of the lower pressure rectification column at a relatively high reboil rate so as to achieve conditions therein close to minimum reflux. To achieve such a high reboil rate, air would need to be condensed in the first reboiler-condenser at a relatively high rate with an attendant high rate of condensation of the air. Introduction of such liquid air into the higher pressure column reduces the rate of formation of liquid nitrogen reflux available to the lower pressure column. As a result, attempts to achieve an adequate argon recovery by increasing the reboil rate beyond a certain limit would become self-defeating.
It is an aim of the present invention to provide a method and apparatus that ameliorate this problem.